Chromosomes: Large coils of DNA. They come in pairs (usually), one from each genetic parent.

Sex Chromosomes: The 23rd pair (assuming there’s two) of chromosomes, X and Y. Typically a “female” will carry XX and a “male” will carry XY.

Gonads: Testes and ovaries. They (usually) make sperm and egg cells respectively.

Genitalia: The external bits, used for coitus (and often other things besides). Generally a penis or vagina/vulva.

Secondary sex characteristics: These are things like breasts, beards, the pitch of one’s voice, and other characteristics that vary based on sex.

Gender: The identity and behaviours (usually) corresponding to one’s sex.

You will have noticed the pattern. There was a lot of “usually” and “typically” in those definitions. Textbooks don’t include “usually.” The medical definitions of sex are rigid, but nature is almost never as simple as we describe it. People identifying as trans, non-binary, or gender diverse contradict with the generally held notion that gender is fixed by one’s physiological sex, and people who are intersex don’t fit within this framework of sex at all. Intersex people may have combinations of chromosomes (such as XXY, XO, XYY etc.), gonads, or genitalia that can’t be placed under a strict “male” or “female” label. A failure to understand diversity means a failure to treat patients properly, and the way trans and intersex patients are treated leaves a lot to be desired.

We all learn at some stage that sex is when a man and a woman put their bits together and that’s how a baby is made. Men have penises, women have vaginas. Men have XY chromosomes and women have XX chromosomes… and that information is as far as it goes for some people. This is the sexual binary model, and it is how most people learn about themselves and each other. Everyone gets taught these “essential facts” about humanity, and they often go uncorrected. In fact, even doctors, for whom a nuanced understanding of sex (both physiology and fun bedroom times) is pretty important, don’t get taught how these things actually work. This ignorance to the diversity of human sex and gender has impacts on trans and intersex people mentally, emotionally, socially, and, when the ignorance extends to the medical field, physically as well.

By defining sex and gender so rigidly, the medical field has found itself describing anything that doesn’t fit within these tight guidelines as an error of development, and in many cases it has resulted in medical mistreatment of patients who do not conform to these standards. Textbook authors assume that the exceptions to the binary model are such a small population that they needn’t be included.

However, that isn’t really the case. While exact figures are hard to come by, an estimate proposed by sexual development expert Anne Fausto-Sterling puts the number of people with intersex bodies at 1% of the population, and 0.6% of US adults identify as transgender, according to a 2016 study from the University of California. Assuming the numbers are similar here, there are nine transgender patients and 15 intersex patients (a person can be both trans and intersex) per doctor working in New Zealand (based on Ministry of Health guidelines that require one doctor to 1500 people). Surely, when it’s people’s wellbeing at stake, those numbers are high enough.

Trans people can often find doctors visits a daunting experience. Doctors are not sufficiently trained to deal with the medical needs of trans patients, even those that aren’t related to being trans. A disturbing amount of anecdotes float around the internet about trans patients who have visited a doctor because of an illness or injury, such as food poisoning or arthritis, and had their doctor blame it on the fact that they were trans and on hormones. This is so common that it has a name: Trans Broken Arm Syndrome. This, along with the fact that a trans person has to let their doctor know that they are trans (leading to medical staff misgendering patients), puts a lot of trans people off seeking medical help when they need it.

The state of “healthcare” for intersex people is worse still. People born with “ambiguous genitalia”, i.e. genitalia that doesn’t look like a typical penis or vagina, have historically been subjected at birth to “normalisation surgery.” During embryonic development, the tissue that forms the genitals usually diverges from something in between a vagina, vulva, and clitoris, and a penis and scrotum. This is thought to be controlled by varying levels of hormones like testosterone and estrogen. This means that genitals exist as a bimodal distribution, to borrow from statistics — the majority of people fall within two major forms of genitals, but there exist things in between.

The people born in between have “ambiguous genitalia”, historically called pseudohermaphroditism, and now referred to by some as Disorders of Sex Development or DSD. For a long time, medical professionals (in the western world) have held the belief that people with ambiguous genitalia must be corrected to a standard “male” or “female”, and perform surgery to assign a sex. The majority of these “normalisations” make the genitals of these newborns into vaginas and vulvas, regardless of the chromosomes or gonads actually possessed. These surgeries often result in scar tissue on the genitals that prevent the person from being able to achieve orgasm from physical stimulation.

The reasoning behind the decision to default to vaginas is that for a healthy sexuality a male should be able to achieve orgasm, but a female need only be receptive. “We recommend a change to the female sex, because the penis was so tiny that a normal sexual life in the male role seemed most unlikely, whereas ‘fertile’ life in the female sex was clearly possible,” reads one article from Lancet in 1984 describing an intersex child of four years of age. Another article describes, “After careful consideration we decided to ease the fulfilment of her female role… a clitoral extirpation and vaginal plastic surgery was performed” — the complete(and unnecessary) removal of an “oversized” clitoris, disregarding the fact that the clitoris is almost essential for orgasm. This is just straight up sexist. These surgeries are often performed on children, even hours-old babies, sometimes without the knowledge of the parents, and almost never with the consent of the person receiving the surgery

Often people who have received normalisation surgery grow up experiencing intense gender dysphoria, having been raised as the gender assigned to them because of the sexassigned by the doctors. This happens regardless of whether the person knows they had received normalisation surgery; the fact they had the surgery is often never revealed to them, as advised to their parents by the doctors. Things are improving, however. Trans and intersex advocacy is reaching the medical field and guidelines are being put in place in many countries to ensure that malpractice does not occur. However, often these are justguidelines — doctors use their own intuition and behave at their own discretion.

For these problems to stop, they really must be addressed at their root: the definition of sex itself. Because medical professionals and biomedical researchers usually work from the rigid and binary definitions described earlier, variation is seen as anomalous, and abhorrent. Binary sex is a model. It is a useful model for understanding sexual reproduction, but it is only a model, and doesn’t explain everything. If sexual development is taught to doctors and scientists in a more nuanced way from the beginning, then trans and intersex people are no longer mistakes of biology, but simply part of normal variation. Scientists both describe and construct reality by creating definitions, and expanding our definitions creates room for trans and intersex bodies to exist within a medical framework, and be treated like any other body should: as a normal human.